The invention relates to a method for preparing poly (p-phenylene) bis (benzoxazolyl) fibers, which belongs to the technical field of preparation of polymer materials. The method comprises the following steps: firstly, composite salt is prepared by diamino hydroxyphenol hydrochloride and a terephthalic acid, and phosphorus pentoxide, a polyphosphoric acid and an auxiliary agent are added into the composite salt, and subjected to prepolymerization under the condition of stirring and heating; secondly, a polymer reaction mixture after prepolymerization is over enters a screw extruder for further polymerization under the pressure of high-pressure inert gas or under the actuation of mechanical propulsion; thirdly, the polymer reaction mixture is extruded by the screw extruder, filtered, accurately measured, ejected, and solidified and formed in a solidification liquid after passing through a section of spinning shaft of inert gas; and fourthly, the polymer reaction mixture is subjected to five procedures of a diluted phosphoric acid aqueous solution, water, hot water, an alkaline solution and purified water for cleaning and removal of a phosphoric acid, and dried, rolled and formed. The method completely removes the phosphoric acid; the process is tempered; drying under the condition of exertion of a tensile force helps to close fiber holes; and the products produced have high strength and modulus.